FIG. 6 shows a simplified side view of the wall tubes forming the wall face of the furnace which composes the combustion chamber of a conventional boiler for thermal power generation.
The combustion chamber of the boiler for thermal power generation is composed of a furnace wall 1 formed by arraying furnace wall tubes 2a for conveying water, steam, or a fluid mixture of them at regular intervals, and welding these furnace wall tubes 2a via membrane bars 3 disposed therebetween (See FIG. 2).
The furnace wall 1 is provided with a furnace wall bottom part A composed of the furnace wall tubes 2a having upward-spiraled fluid passages; a nose part C which has nose wall tubes 5a disposed in a middle part of a furnace rear wall B adjoining the furnace wall bottom part A with a side view resembling a sidewise V (<); and a screen part D having screen tubes 7.
There are also plural burners 4 provided for supplying fuel from outside for combustion, which are arrayed in each of the plural stages provided in the vertical direction at corresponding positions on the lower side of the front wall and rear wall of the gas flow of the furnace wall 1. These burners 4 heat the fluid inside the furnace wall tubes 2a and make it move upwards from the furnace wall bottom part A inside the inclined furnace wall tubes 2a. 
The fluid heated by the burners 4 receives a different amount of heat depending on the arrayed position of the furnace wall tube 2a provided for conveying the fluid, and on the positional relationship between the furnace wall tube 2a and the burners 4. Therefore, in order to make the amount of heat received by the fluid uniform, regardless of the arrayed position of the furnace wall tube 2a and the positional relationship between the furnace wall tube 2a and the burners 4, the furnace wall tubes 2a in the furnace wall bottom part A are upward-spiraled. Such a structure of the upward-spiraled furnace wall tubes 2a of the conventional boilers for thermal power generation is disclosed in Japanese Published Unexamined Patent Application No. 2000-130701, paragraph [0027].
FIG. 7 and FIG. 8 (as viewed from the direction of the lines II—II of FIG. 7) show a detailed structure of the connection part (hereinafter also referred to as the transition part) between the spiral furnace wall tubes 2a in the furnace rear wall, and the nose wall tubes 5a and the screen tubes 7.
The combustion gas G in the furnace, as shown in FIG. 6, rises from the furnace wall bottom part A; turns at the nose part C to the left side on the drawing; passes through the furnace ceiling part; and then flows towards an unillustrated furnace rear heat transfer part. Thus, the combustion gas Grises while making a detour in the upper part of the furnace wall 1. In contrast, if the nose part C is absent, the combustion gas G generated at the burners 4 region at the furnace wall bottom part A flows towards the right side on FIG. 6; passes through the furnace ceiling part; and flows towards the unillustrated furnace rear heat transfer part. Without the nose part C, the combustion gas G flows the shortest route in the furnace wall 1 in this manner, which shortens the retention time of the combustion gas G in the furnace, thereby making the combustion of the fuel insufficient. The shortened retention time of the combustion gas G in the furnace also makes the heat storing insufficient in the furnace wall tubes 2a and the other heat transfer tube regions in the furnace, thereby causing high-temperature combustion gas G to flow to the furnace rear heat transfer part side. The high-temperature combustion gas G causes the heat transfer tubes arranged on the furnace rear heat transfer part to have clinkers or slag, which are difficult to remove after being hardened.
This makes it necessary to provide the nose part C which must have a complicated tubing structure. The terminal parts of the spiral furnace wall tubes 2a are positioned in the intermediate part of the nose part C composed of the nose wall tubes 5a and others. Consequently, the header 6 for adjusting the number of tubes and mixing the inner fluid, which is required in the connection part (transition part) between the spirally inclined furnace wall tubes 2a and the screen tubes 7 because of the difference in number between the furnace wall tubes 2a and the nose wall tubes 5a, is conventionally disposed inside the nose part C as shown in FIG. 7.
Other furnace wall tubes 2b, which extend upright from the inclined terminal parts of the furnace wall tubes 2a whose fluid passages are upward-spiraled, are connected with the header 6. Then the header makes the fluid flow towards the nose wall tubes 5a. Between the header 6 and the nose wall tube 5a are provided fluid passages 5f for conveying the inner fluid downwards. The fluid passages 5f are arranged in parallel with the vertical furnace wall tubes 2b. 
In the transition part, the inclined terminal parts of the furnace wall tubes 2a are directly connected with the screen tubes 7, which are composed of thick tubes with higher rigidity than the furnace wall tubes 2a so as to support the weight of the furnace wall bottom part A by a small number. However, it is impossible to transfer the weight of the furnace wall bottom part A to the screen tubes 7 only by the furnace wall tubes 2a with insufficient rigidity. Therefore, there are reinforcing supports 8 provided between the furnace wall tubes 2a and the screen tubes 7 in order to compensate for the rigidity of the furnace wall tubes 2a and to transfer the weight of the furnace wall bottom part A to the screen tubes 7.
According to the aforementioned prior art, since the terminal parts of the spirally inclined furnace wall tubes 2a are located in the intermediate part of the nose part C, the header 6 is provided to compensate for the difference in number between the furnace wall tubes 2a and the nose wall tubes 5a and to mix the inner fluid. The header 6 is installed inside the nose part C, and the inner fluid coming out of the header 6 flows through fluid passages 5f into the nose wall tubes 5a whose side views resembles a sidewise V (<).
Thus in the conventional furnace wall structure, the water inside the fluid passages 5f located lower than the header 6 cannot be drained while the operation of the boiler is suspended.
Furthermore, according to the prior art, the reinforcing supports 8 must be installed in the screen tubes 7 that are directly connected with the spirally inclined furnace wall tubes 2a, and such a complicated structure leads to a cost increase.
The object of the present invention is to provide a furnace wall structure which can drain the water inside the nose wall tubes while the operation of the boiler is suspended, and also to provide a furnace wall structure which can dispense with the reinforcing supports for supporting the weight of the furnace wall bottom part.